The present invention relates generally to signal reception systems and methods for free space communication, and more particularly to signal reception systems and methods for free space communication with compensation for ambient noise signals.
Photo detectors can be used to detect laser signals to support high-speed data communications for a variety of applications, such as for ground-to-air, air-to-air and other types of Unmanned Aerial Vehicle (UAV) communications, deep space communications, and underwater submersible communications. However, unwanted ambient light (or other noise signals) often causes the photo detector to saturate, causing an undesirable signal-to-noise (S/N) ratio. Saturation is generally not a problem during nighttime conditions, when ambient light levels are low. However, during daytime conditions, sunlight can produce a direct current (DC) photo current and cause the photo detector to saturate, making a desired communication signal (e.g., laser beam signal) undetectable. Moreover, in deep space application, sunlight and other atmospheric conditions can produce similar saturation problems. Filters are sometimes used to reduce saturation, including optical filters, colored and neutral density filters, and low, band pass, and high pass filters. However, known filters have not performed well. Some receivers don't have any kind of filtering, and use computers to sift through all the signals to receive the correct one. Computerized signal analysis has drawbacks, such as requiring complex equipment and software. All of these filtering and computer processing methods have the effect of reducing signals from unwanted bandwidths while keeping others, and when they are placed with photo-diodes, are often called ‘daylight’ filters. None of these known methods truly cancels out all unwanted ambient signals; they only diminish the power of signals in unwanted bandwidths.
Known optical laser receivers, in their most basic construction, consist of one photo-detector. Such a known communication system is illustrated in FIG. 1, which shows laser transmitter 10 having a laser illustrated as diode D1 and a receiver 12 having a single optical receiver element illustrated as photo detector diode D2. In the absence of any kind of filtering, sunlight received by the photo diode D2 of the receiver 12 will produce a strong DC current. Strong sunlight will indeed overpower a laser signal from the diode D1, which may have been transmitted a great distance away from the receiver 12. An output voltage VO of the system of FIG. 1 can be represented by Equation 1, where IA is current produced by ambient noise such as ambient light (e.g., sunlight) and any and all other received ambient noise, IL is current produced by a signal from the transmitter 10, and R2 is the resistance of a resistor R2 in the receiver 12:VO=(IA+IL)●R2  (1)
Discerning a laser signal is difficult amid all the background noise received by the photo-detector diode D2. Known filters and computerized processing methods are simply not good enough to eliminate all the unwanted noise, including sunlight. As a result, some believe that these saturation problems mean optical free space communication system are not feasible for many applications, such as for UAV communications.
Thus, additional techniques are needed to further reduce problems of atmospheric or other ambient noise in communication signal reception.